Fiber Laser Resonators

In order to form a laser resonator with fibers, one either needs some kind of reflector (mirror) to form a linear resonator, or one builds a fiber ring laser.
Various types of mirrors are used in linear fiber laser resonators:

Figure 1:
A simple erbium-doped femtosecond laser, where the Fresnel reflection from a fiber end is used for output coupling.

In simple laboratory setups, ordinary dielectric mirrors can be butted to the perpendicularly cleaved fiber ends, as shown in Figure 5.
This approach, however, is not very practical for mass fabrication and not very durable either.

A better power-handling capability is achieved by collimating the light exiting the fiber with a lens and reflecting it back with a dielectric mirror (Figure 3).
The intensities on the mirror are then greatly reduced due to the much larger beam area.
However, slight misalignment can cause substantial reflection losses, and the additional Fresnel reflection at the fiber end can introduce filter effects and the like.
The latter effects can be suppressed by using angle-cleaved fiber ends, which however introduce polarization-dependent losses.

There are many different kinds of fiber lasers, some of which are discussed in the following.

Figure 5:
Setup of a simple fiber laser. Pump light is launched from the left-hand side through a dichroic mirror into the core of the doped fiber. The generated laser light is extracted on the right-hand side.

High-power Fiber Lasers

Whereas the first fiber lasers could deliver only a few milliwatts of output power, there are now high-power fiber lasers with output powers of hundreds of watts, sometimes even several kilowatts from a single fiber.
This potential arises from a high surface-to-volume ratio (avoiding excessive heating) and the guiding effect, which avoids thermo-optical problems even under conditions of significant heating.

Nowadays, high-power fiber lasers are widely used e.g. in laser material processing.
Examples for processes are laser welding and laser cutting e.g. on metals, but also with many other industrial materials.
Many applications use a fiber laser machine with continuous-wave operation; limitations concerning pulse generation e.g. with Q switching are substantial, so that bulk lasers reach clearly superior performance in such domains.

Upconversion Fiber Lasers

Figure 6:
Level scheme of thulium (Tm3+) ions in ZBLAN fiber, showing how excitation with an 1140-nm laser can lead to blue fluorescence and laser emission.

The fiber laser concept is most suitable for the realization of upconversion lasers, as these often have to operate on relatively “difficult” laser transitions, requiring high pump intensities.
In a fiber laser, such high pump intensities can be easily maintained over a long length, so that the gain efficiency achievable often makes it easy to operate even on low-gain transitions.

In most cases, silica glass is not suitable for upconversion fiber lasers, because the upconversion scheme requires relatively long lifetimes of intermediate electronic levels, and such lifetimes are often very small in silica fibers due to the relatively large phonon energy of silica glass (→ multi-phonon transitions).
Therefore, one mostly uses certain heavy-metal fluoride fibers such as ZBLAN (a fluorozirconate) with low phonon energies.

The probably most popular upconversion fiber lasers are based on thulium-doped fibers for blue light generation (Figure 6), praseodymium-doped lasers (possibly with ytterbium codoping) for red, orange, green or blue output, and green erbium-doped lasers.

Narrow-linewidth Fiber Lasers

Fiber lasers can be constructed to operate on a single longitudinal mode (→ single-frequency lasers, single-mode operation) with a very narrow linewidth of a few kilohertz or even below 1 kHz.
In order to achieve long-term stable single-frequency operation without excessive requirements concerning temperature stability, one usually has to keep the laser resonator relatively short (e.g. of the order of 5 cm), even though a longer resonator would in principle allow for even lower phase noise and a correspondingly smaller linewidth.
The fiber ends have narrow-bandwidth fiber Bragg gratings (→ distributed Bragg reflector lasers, DBR fiber lasers), selecting a single resonator mode.
Typical output powers are a few milliwatts to some tens of milliwatts, although single-frequency fiber lasers with up to roughly 1 W output power have also been demonstrated.

An extreme form is the distributed-feedback laser (DFB laser), where the whole laser resonator is contained in a fiber Bragg grating with a phase shift in the middle.
Here, the resonator is fairly short, which can compromise the output power and linewidth, but single-frequency operation is very stable.

Of course, further amplification to much higher power levels in a fiber amplifier is possible.

Q-switched Fiber Lasers

Figure 7:
Simple Q-switched fiber laser. The setup looks exactly the same as that of a mode-locked laser as shown above (Figure 2), but the SESAM parameters are different.

Due to the high laser gain, the details of Q switching a fiber laser are often qualitatively different from those of a bulk laser, and more complicated.
One often obtains a temporal sub-structure with multiple sharp spikes, and there is a possibility of producing Q-switched pulses with a duration well below the (typically long) resonator round-trip time.

Raman Fiber Lasers

A special type of fiber lasers are fiber Raman lasers, relying on Raman gain associated with the fiber nonlinearity.
Such lasers usually use relatively long fibers, sometimes of a type with increased nonlinearity, and typical pump powers of the order of 1 W.
With several nested pairs of fiber Bragg gratings, the Raman conversion can be done in several steps, bridging hundreds of nanometers between the pump and output wavelength.
Raman fiber lasers can e.g. be pumped in the 1-μm region and generate 1.4-μm light as required for pumping 1.5-μm erbium-doped fiber amplifiers.

Fiber Lasers with Semiconductor Optical Amplifiers

There are some lasers which have a semiconductor optical amplifier (SOA) as the gain medium in a resonator made of fibers.
Even though the actual laser process does not occur in a fiber, such fibers are sometimes called fiber lasers.
They typically emit relatively small optical powers of a few milliwatts or even less.
Sometimes they exploit the very different properties of the semiconductor gain medium, as compared with a rare-earth-doped fiber, in particular the much smaller saturation energy and upper-state lifetime.
Rather than only generating coherent light, such lasers can be used for information processing in optical fiber communications systems – for example the wavelength conversion of data channels based on cross-saturation effects.

Special Attractions of Fibers as Laser Gain Media

As fibers can be coiled and the light propagating in fibers is well shielded from the environment (e.g. concerning dust), fiber lasers can have a compact and rugged setup, provided that the whole laser resonator is built only with fiber components (all-fiber setup) such as fiber Bragg gratings and fiber couplers (i.e., avoiding free-space optics and any requirement for alignment).

Fiber gain media have a large gain bandwidth due to strongly broadened laser transitions in glasses, permitting wide wavelength tuning ranges and/or the generation of ultrashort pulses.
Also, fiber lasers have broad spectral regions with good pump absorption, making the exact pump wavelength uncritical, so that temperature stabilization of the pump diodes is usually not necessary.

Fibers have a limited gain and pump absorption per unit length, making it difficult to realize very short resonators e.g. for single-frequency lasers or for multi-gigahertz mode-locked lasers.
However, significant progress has been made in this direction recently via the development of very highly doped fibers, usually made from phosphate glasses.

For these reasons, the operation details of a fiber laser (or fiber laser system) can often not be understood only based on simple analytical calculations.
Numerical simulations, carried out with some kind of fiber simulation software, are therefore required for calculating the possible laser performance, analyzing detrimental effects, and optimizing prototype and product designs.
Such simulations can address many different technical aspects:

A mode solver, i.e., a calculator for fiber modes, can produce inputs for further calculations – in particular, mode intensity profiles.

In some situations, numerical beam propagation is of interest.
For example, this is often the case for highly multimode fibers, including the pump claddings of double-clad fibers.

Refined algorithms are required for calculating the steady state of lasers and amplifiers, with a self-consistent solution for optical intensities and excitation densities of laser-active ions throughout the fiber.
(Note that optical intensities and excitation densities mutually influence each other.)

Dynamical models are used for calculating pulse amplification and Q switching, for example.

As an example for surprising features even of simple fiber lasers, Figure 9 shows the optical powers and excitation densities along the fiber of an Yb-doped single-mode fiber laser.
A fiber Bragg grating with 25% peak reflectivity at 1030 nm on the right side serves as the output coupler, whereas a highly reflecting Bragg grating is used on the left side.
The pump light (at 975 nm) is coupled in through that grating.
A nearly linear (rather than exponential) decay of pump power on the left side results from strong pump saturation.
The fiber is somewhat over-long, resulting in slight signal reabsorption on the right side.
That reabsorption maintains a significant ytterbium excitation despite the vanishing pump power, but causes only a negligible reduction in signal output power.
Losses via ASE (not shown here) are also negligible.

Figure 9:
Optical powers and excitation densities along the fiber of an Yb-doped single-mode fiber laser, core-pumped at 975 nm.
Note that the intracavity signal power can be higher than the pump power; only part of that power can be coupled out.
The simulation has been done with the software RP Fiber Power.

Figure 10 shows the same for a modified output coupler grating, so that lasing occurs at 1080 nm.
The lower emission cross-sections at 1080 nm lead to a higher degree of Yb excitation and thus to weaker pump absorption.
This demonstrates that the required fiber length depends not only on the absorption characteristics at the pump wavelength, but also on the details for the signal, such as the signal wavelength and the resonator losses.

Figure 10:
Same as in Figure 9, but for a fiber Bragg grating for lasing at 1080 nm.

If the fiber length in the last case would be reduced to 0.7 m, one might expect a moderate reduction in output power due to incomplete pump absorption.
However, a simulation (not shown here) tells that lasing would stop completely, and 94% of the pump power would leave the fiber on the right side.
The Yb excitation density of about 50% throughout the fiber would not be sufficient to reach the laser threshold.
For a reduced pump wavelength of 940 nm, however, lasing would be possible again – despite the reduced pump absorption cross section, because pump saturation effects would be weaker.

Suppliers

The Koheras narrow linewidth, single-frequency fiber lasers are ultra-low noise sources with longitudinal single mode and single frequency operation. The lasers are based on a DFB design ensuring robust and reliable operation and are delivered as fully integrated systems for industrial and scientific applications. Koheras offers an unprecedented low phase- and intensity noise level at Yb, Er and Tm wavelengths. It has a very high stability, and mode-hop free inherent single frequency output – even when exposed to changing environmental conditions. You can also get shot noise limited solutions for applications demanding an extra low intensity noise level.

Menlo Systems' femtosecond fiber lasers based on Menlo figure 9® patented laser technology are unique in regard to user-friendliness and robustness. We offer solutions for scientific research as well as laser models engineered for OEM integration. From the shortest pulses to highest average power beyond 10 Watts and pulse energy beyond 10 μJ, we have the solution for your application ranging from basic research to industrial applications in spectroscopy, quality control, and material processing.

The new LightWire series Ultrafast fiber lasers feature turn-key operation, monolithic all-in-fiber design and require no maintenance. Femtosecond and picosecond pulse durations are available for research and industrial applications.

The key for successful integration of ultrafast technology are robust, cost-effective systems with simple push-button operation. TOPTICA offers several products fulfilling these requirements: ultrafast fiber lasers based on erbium (Er) and ytterbium (Yb) like the FemtoFiber smart, FemtoFiber pro, FemtoFiber ultra and FemtoFiber dichro series. All these systems are based on TOPTICA FemtoFiber technology.

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection:

(enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you enter any personal data, this implies that you agree with storing it; we will use it only for the purpose of improving our website and possibly giving you a response; see also our declaration of data privacy.